A mutant of Escherichia coli K-12 carrying a single mutation, designated rfaD, which affects the synthesis of the aldoheptose, L-glycero-D-mannoheptose, has been isolated. Phenotypically this mutant is novobiocin-hypersensitive with altered lipopolysaccharide (LPS) core biosynthesis. It was observed that the LPS of the rfaD mutat contained the aldoheptose D-glycero-D-mannoheptose rather than the normal L-glycero-D-mannoheptose. It is postulated that L-glycero-D-mannoheptose is formed from d-glycero-D-mannoheptose via a racemization. A crude assay to measure the epimerase activity has been developed. The following facts have been established: (i) the rfaD gene product, ADP-L-glcero-D-mannoheptose-6-epimerase, is necessary for the conversion of ADP-D-glycero-D-mannoheptose to ADP-L-glycero-D-mannoheptose. (ii) The nucleotide ADP-D-glycero-D-mannoheptose accumulates in rfaD mutant strains. Two chimeric colicin El plasmids carrying the coding sequence for therfaD wild type allele have been selected, subcloned, and shown to complement the rfaD phenotype. These clones also result in an amplification of ADP-L-glycero-D-mannoheptose-6-epimerase activity.